The invention belongs to the technical field of ocean chemical engineering, and relates to a method for efficiently preparing high deacetylation degree chitosan under a condition of relatively low alkaline concentration. The method comprises the following steps: firstly adopting alkali liquor with the mass fraction of 10% to 25% to carry out soaking pretreatment on chitin, then carrying out deacetylation under negative pressure, and carrying out post-treatment to obtain the chitosan. The method for preparing the chitosan with the deacetylation degree of 95% or more than 95% by directly adopting the chitin under a relatively low alkaline liquor concentration is realized, and the reuse of the alkaline liquor is realized. Compared with the prior art, the production efficiency is improved by two to three times, no liquid waste is generated in the process, and the clean production of chitosan is realized.